I have a dozen flights on my version of the Laser type and things are progressing well. Running a Whirlwind 200AC in front of a straight valve ECI Titan IO360 with a Superior cold air intake. I had the prop dynamically balanced but I'm still getting a shake in the airframe. My guess is around 5-6Hz. This happens at 2440 rpm and changes a little but does not go away at lower rpm. Above that I would say the vibration is normal 4cyl. Prop is clocked at 10/4 as viewed from front at #1 TC. Looked for the obvious but nothing looks weird. Whirlwind says to balance it on the governor instead of on the pitch stops. Looking for more ideas to try when I'm back in town.

Guys, look for timing and fouled plugs. I do plenty airshows in China and my Barret 540 tends to foul the plugs at ground ops with the crapy AVGAS 100 (no LL). A full flight at max power or change/clean plugs solves the vibrations. By the way, I have dinamicaly balanced Claw to have it "smothed" every time I get vibs is from plugs now

Here is the latest. Tried different length bushings in rubber mounts, no change. Backed off the timing to 22 degrees, no change. Cleaned and tested the plugs, they looked good but did it anyway, no change. Mag drop during run up has always been 30-40 rpm and no real difference in vibration during a mag check in flight. Looked very hard for any signs of grounding between the "moving" engine and all the stationary stuff. Nothing I can see. Now the weird part. Flipped the prop 180 degrees, way smoother. Still a slight wobble or shake at 2430ish and lower, but noticeably smoother at the higher rpms. Then, realizing I left the balance weight on from the previous dynamic balance I took it off and the shake was back to how it was. So balanced it again down to .03ips and now it's worse than before!! Also checked mags inflight and nothing weird. I'm now out of ideas and the weather is most likely going to prevent more testing. Oh and the balancing was done useing two different Aces machines.

Put the weight back on. You had it right for one moment there. Go back. It's not a crime to put it back, them fly and change the weight or move it a bit. Would be better if you could fly with the Aces Machine.
Dynamic balancing on the ground misses the mark in a whole lot of ways.
First, you will never get the pitch the same as 'in flight'
Second, airframe sympathetic vibrations will be different when the aircraft is supported by the wings instead of the wheels.
My son's Giles G-200 has vibrations with an MT prop, freshly overhauled. He has balanced it, but there are some RPM & RPM Airspeed combinations that shake. Doesn't really matter in competition, but a few hours of cross country flight has him finding the smooth spot that fits the speed/altitude he needs.